Notes and discussion around my journey toward a Cisco Certified Internetworking Expert (CCIE) Routing and Switching Certification. The views expressed here are my own and do not represent those of my employer, or Cisco, or indeed any other organisation, person or entity! The information here is not guaranteed to be accurate and I will not be held responsible for the consequences of its use. Discussion is welcomed; abuse really is not, and I reserve the right to redact comments.

Sunday, 28 June 2015

Multi-Layer Switch: routed port, switchport and SVIs

'switchport'

The 'switchport' command tells the switch (usually a Multi-Layer Switch or MLS) to treat the port as a layer 2 port, i.e. as a member of a VLAN and to allow it to switch frames and learn MAC addresses etc., as well as participating in all other layer 2 processes such as spanning-tree.

'no switchport'

The 'no switchport' command tells the switch to treat the port as a layer 3 interface, so that you can run a routing protocol, add an interface IP address (or other layer 3 address) and create sub-interfaces, none of which is possible on a layer 2 interface. If you try running this command on a layer 2 only switch (e.g. a 2950) it will not understand it and reject it as 'incomplete', as shown below:

A routed port does not belong to a VLAN as far as the MLS is concerned because it has no concept of VLANs at layer 3, just a like a port on a router. However, on a MLS each VLAN also has a layer 3 interface: the VLAN interface, also known as an SVI. This is created on an MLS when the VLAN itself is created.

On a pure layer 2 switch, such as the 2950, there is only one layer 3 interface: this is the 'VLAN1' interface (an SVI) that you configure to allow management connectivity.